1. Field of the Invention
The present invention relates to a plasma CVD device and a discharge electrode.
2. Description of the Related Art
Recently, in view of mass production of electronic devices such as a thin film solar battery, a low-cost manufacturing process has been expected to be developed. Above all, in a semiconductor layer formation process, efforts have been made to increase a utilization efficiency of a film formation gas, to use a high-frequency power source for causing discharge to perform high-speed film formation, and the like. As compared with a hard substrate such as a glass substrate, a soft substrate such as a thin resin film has enough flexibility to be rolled up. As one means of reducing the manufacture cost using such a flexible substrate, a method of continuously conducting unit operations such as film formation, printing and laser processing in an inline manner while rewinding a rolled-up flexible substrate around another roll is known. This method is referred to as a Roll-to-Roll method.
As a method of particularly enhancing the productivity of a thin film formation process, it is effective to perform continuous conveyance and film formation using a film formation apparatus equipped with a conveyor employing a Roll-to-Roll method as described in, for example, Japanese Patent Application Laid-Open Nos. Sho 58-216475 and 59-34668. In the film formation apparatus equipped with a conveyor employing a Roll-to-Roll method, continuous film formation is performed while continuously conveying a flexible film substrate. In order to efficiently obtain a desired thickness, there are methods such as extending the length of a discharge electrode for film formation, increasing the carrying speed, and continuously forming a long shaped film.
In the case where a non-single crystalline silicon film is to be formed by a plasma CVD method, a silane gas SiH4 is decomposed in a discharge space to reach a surface of the non-single crystalline silicon film on a substrate so as to be bonded therewith. As a result, a film is formed. In this film formation process, the silane gas in the discharge space after decomposition causes cohesion among monomolecules even before reaching the surface of the non-single crystalline silicon film on the substrate. As a result, there are cohered particles diversely called according to their sizes, such as a material gas of monomolecules, a monomer, a cluster in which a plurality of molecules are cohered to each other, a nuclear, and a ultrafine particle. The cohered particles generated in a discharge space are called herein fine particles. On the other hand, a non-single crystalline silicon film is formed on a discharge electrode and on a wall of a vacuum chamber in addition to the substrate on which a film is to be formed. Thereafter, the non-single crystalline silicon film exfoliates due to the difference in adherence or in stress with the wall or the electrode, resulting in fragmental particles. The particles generated due to exfoliation of the film after its formation on the wall or the electrode are herein referred to as fragmental particles.
In the case where electronic devices such as a solar battery are to be formed, if fine particles or fragmental particles having a diameter larger than a desired thickness of the non-single crystalline silicon film adhere onto a substrate on which a film is to be formed, the characteristics of the solar battery and a yield of non-defective products are lowered. FIGS. 1A through 1C are cross-sectional views showing the process of forming a solar battery, with which the reason of a lowered yield will be described. First, a lower electrode layer 102 is formed on a substrate 101 on which a film is to be formed, and a non-single crystalline silicon layer is formed thereon. During the formation of the non-single crystalline silicon layer, fine particles 104 and fragmental particles 105 described above adhere onto the lower electrode 102 to be introduced into the non-single crystalline silicon layer. The sizes of the fine particles 104 and the fragmental particles 105 are varied; the fine particles 104 or the fragmental particles 105 that are larger than a thickness of the non-single crystalline silicon layer are also present. The fine particles 104 or the fragmental particles 105 may fall off after the formation of the non-single crystalline silicon layer. FIG. 1B shows holes 106 formed after the fine particles 104 or the fragmental particles 105 fell from the non-single crystalline silicon layer. Thereafter, an upper electrode layer 107 is formed. In the hole regions formed after the fine particles 104 and the fragmental particles 105 fell off, regions 108 where the upper electrode and the lower electrode contact each other are formed. Since the contact regions 108 are extremely small and have high resistance, a leak current in these regions 108 is extremely small. In the case where the solar battery is under solar light of AM 1.5, a leak current hardly affects the output characteristics of the solar battery. In the case where the solar battery is under light having low illuminance such as light from a fluorescent lamp, however, a leak current affects the output characteristics of the solar battery to lower the characteristics and a yield of the solar battery.
FIGS. 2A and 2B show a discharge electrode having a conventional structure for a plasma CVD device. Discharge is caused between a ground electrode 202 and a high-frequency electric power side electrode 203 to form a film on a substrate 201 on which a film is to be formed (hereinafter, referred to simply as substrate 201). A material gas 206 passes through the high-frequency electric power side electrode 203 to jet out from small holes formed on an electrode substrate 294, resulting in a gas flow 207 flowing in the direction of the substrate 201. Since the electrode substrate 204 is a metallic plate having a plurality of small holes formed therethrough, the electrode substrate 204 is also referred to as a shower plate. A discharge electrode having the gas introducing structure as described above is herein referred to as a shower plate type discharge electrode. In a shower plate type discharge electrode, fine particles 211 grown in a discharge space 205 and fragmental particles 210 exfoliating from the electrode plate 204 are subjected to viscous resistance from the gas flow 207 flowing in the direction of the substrate to flow in a direction along the substrate 201. As a result, the fine particles 211 and the fragmental particles 210 adhere onto the substrate 201.
If a film is formed at high speed using a plasma CVD method or the like, the probability of generation of fine particles grown from a material gas becomes high in a sheath region in a discharge space. Moreover, if continuous film formation is performed over a long period of time, a film deposited on a discharge electrode exfoliates as fragmental particles to adhere onto the substrate. A thickness of the film deposited on the discharge electrode increases with elapse of film formation time, whereby the probability that fragmental particles adhere onto the substrate becomes higher.
As one of the methods of preventing fragmental particles that exfoliate from the discharge electrode from adhering onto a substrate on which a film is to be formed, a film on the surface of the electrode is removed by etching before the film exfoliates in the state where the film is deposited to some degree on the surface of the electrode. In practice, however, when a film is formed by using a Roll-to-Roll method, for example, etching should be frequently conducted before the film is continuously formed over a sufficient length. Therefore, in order to continuously form a film, the film formation process must be often interrupted. The employment of a method of frequently conducting etching to prevent fragmental particles from adhering results in a poor production efficiency. Although a method of heating an electrode plate or the like may be used to restrain the occurrence of exfoliation of the film from the discharge electrode, there is still a need of conducting etching before exfoliation of the film. Accordingly, it is not possible to continuously form a film over a sufficiently long period of time.
In order to remove fine particles present in a discharge space, there is a method of causing a material gas flow in a direction parallel to a substrate on which a film is to be formed, as disclosed in Japanese Examined Patent Application Laid-Open No. Sho 62-43554. FIG. 3 shows a material gas flow in the case where a material gas is flowed in a direction parallel to a substrate on which a film is to be formed. In this method, a gas flow parallel to a substrate 301 on which a film is to be formed (hereinafter, referred to simply as substrate 301) gradually contains a flow 306 toward the substrate 301 due to turbulence of the gas flow while moving over a long distance between the substrate 301 and a discharge electrode 303. Fine particles generated in a discharge space 304 or fragmental particles generated by exfoliation of the film deposited on the discharge electrode 303 move along the gas flow. A part of the particles flow in the direction of the substrate 301 due to turbulence or diffusion of the gas flow to adhere onto the substrate 301. Moreover, as disclosed in Japanese Patent Application Laid-Open No. Hei 5-144595, there is also a method of introducing a gas flow from one direction of an enclosed space containing a discharge electrode and exhausting the gas flow from another direction. Also in this method, since fragmental particles and fine particles move along the gas flow over a long distance between a substrate and a film formation surface opposing thereto, a part of the particles flow in the direction of the substrate on which a film is to be formed due to turbulence or diffusion of the gas flow to adhere thereto.
A pulse plasma method is for interrupting discharge once before reactive monomolecules generated by decomposition in a discharge space cohere to each other and grow to have the size of fine particles, so that relatively small fine particles can be exhausted along the flow of a material gas. The electric power from a power source for discharge is pulsed because an ON state and an OFF state are alternatively repeated in a short period of time. In the pulse plasma method, however, when it is attempted to exhaust fine particles having the size that does not lower the characteristics of a solar battery or the like, a period of discharge time becomes extremely short and a time period in which discharge is interrupted becomes relatively long. As a result, a utilization efficiency of a material gas is lowered. In addition, since a gas flow is present even in the period where discharge is interrupted, fragmental particles adhere onto the substrate on which a film is to be formed.